They decided against making this set a Base set, and chose it as an Expansion instead, primarily due to the card series ending (before moving to GT), and the lack of significant personalities available to be used as MPs
While the card is naturally being run alongside the new Edge of Eternities Commanders, Ragost, Kilo, etc., for thematic reasons, its also part of a number of powerful, well-established decks in the format
Since most Hydras don't have generic costs to reduce, this mana is instead automatically pumped into the X section of a Hydra's mana cost, essentially giving it a free power and toughness boost
Its only commonality is having 0 ATK, which Ghost Belle and Ash Blossom both share